1. Field of the Invention
This invention relates generally to improvements in valves and, more particularly, but not by way of limitation, to improvements in ball valves.
2. Discussion of the Prior Art
An ever present danger in many industrial plants is the possibility that a fire may break out in the plant. While appropriate preventive measures are a normal part of plant operations, fires nevertheless occur with a frequency such that, where possible, it is desirable to construct mechanical devices utilized to carry out the activities for which the plant is established to limit the damage which might be occasioned by a fire. In particular, valves and conduits used to transport fluid about the plant are constructed with fire safety in mind.
The problems associated with the exposure of valves to fire can take several forms. Thus, for example, seals formed of organic materials can be destroyed so that the valve loses the capacity to obstuct fluid flow through a conduit wherein the valve is disposed. When the valve is disposed in a coduit which transports a flammable fluid and where the conduit leads to the source of fire, the failure of the valve can result in a continuous feeding of the flames so that it can be very difficult to bring the fire under control. A valve which is designed to close despite destruction of organic seals is disclosed in the applicant's co-pending United States Patent Application entitled "Fire Safe Disc Valve", Ser. No. 865,853, filed Dec. 3, 1977.
Seals need not be destroyed for the hazards of fire to be exacerbated by the effect of fire on a valve. A common valve construction includes a body having a valve chamber which contains a rotatable valve member and which includes annular seal assemblies encircling passages leading to the valve chamber and mating with the body and the valve member to disrupt fluid flow through the valve when the valve member is rotated to a preselected position. The seal assemblies form seals with the body and with the valve member so that the valve member blocks each passage into the valve chamber. While such construction provides an effective mode of valve closure, it also results in the formation of a trapped fluid chamber, forming a portion of the valve chamber, about the valve member. That is, when the valve is closed, the seal assemblies isolate a portion of the valve chamber between the seal assemblies from the passages leading to the valve chamber. If such a valve is subjected to heat, both the body of the valve and fluid in the trapped fluid chamber will expand. Where the rate of expansion of the fluid with temperature exceeds the rate of expansion of the body, the valve body is subjected to large internal pressures. In extreme cases, such pressures can be sufficient to cause a rupture of the body of the valve. The damage caused by such reupture can, in many situations, not be limited to the destruction of the valve. Where the heating of the valve is caused by a fire and where the fluid is flammable, such a rupture will have the effect of adding a new source of combustible material to the fire, and, in some cases, of spreading the fire.
It is common practice to provide the valve chamber of such valves with pressure relief means to prevent rupture via heating of the valve. In particular, it is conventional to construct valves such that a high pressure in portions of the valve chamber between seal assemblies will cause one or both of the seal assemblies to displace away from the valve member to vent such pressure into one or both of the passages leading to the valve chamber.
This conventional mode of providing pressure relief to isolated portions of a valve chamber is subject to an important limitation. If, for some reason, the seal assemblies should bind in the body of the valve, the pressure relieving features of the valve become inoperative and heating of the valve can result in the above-described problem despite the presence of the relief features. Unfortunately, a number of factors, acting singly or in combination, can cause the seal assemblies to bind. In particular, a seal assembly will generally be supported on an annular surface formed about the passage encircled by the assembly and forming a bearing surface for the seal assembly. The periphery of the seal assembly forms a mating bearing surface so that axial movement of the seal assembly occurs by sliding of the periphery of the seal assembly along the annular surface formed about the passage to support the seal assembly. These surfaces are subject to corrosion which can bind the seal assemblies. Moreover, foreign matter can become lodged between these surfaces to bind the seal assembly and the seal assemblies and/or body can become distorted to produce binding.